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Mattski created the topic: Calculating density
Hi can someone work out this calculating density height equation for me? Having problems understanding it.
Looking at the density altitude equation, PA is 1380ft which could be rounded to 1500ft at +26 degrees C at indicated
So if MSL is +15 degrees C and we assume +1 degree decrease in temp per 500ft of altitude, so 1500ft should be +12 deg C
Would that make it a +14 degree Isa day giving us a DA ( 14 x 120 = 1680) + 1380 = 3080ft
Am I doing the calculations wrong?
I'm doing some revision as I'm getting closer to my chopper flight exam.
Good catch Dobbo - in Stuart's calculation of ISA temperature he may have accidentally used '26' rather than '15'.
Here is the corrected working out in extended form
Altitude: 1800 ft
QNH: 1027 hPa
Temperature: 26 degrees c
Pressure Height = altitude + 30(1013-QNH) = 1800 + 30(1013-1027) = 1800 + 30(-14) = 1380
ISA Temp = 15 - 2(1500/1000) = 15 - 3 = +12
ISA Deviation = OAT - ISA Temp = 26 - 12 = plus 14
Density height = Pressure Height + 120(ISA Deviation) = 1380 - 120(14) = 3060 feet
You can also use a flight computer - just work out the pressure height and set the temperature against pressure height and read the value in the density altitude window. Note it might read 3000 ft - near enough, good enough